With the extension of human life, cancer emerged as the leading cause of death in recent years. “2012 Chinese Cancer Registry Annual Report” shows that about 3.5 million of new cancer cases occur and about 2.5 million of persons die of cancer each year in China. Lung cancer has a highest incidence among malignant tumors in China, followed by stomach cancer, colorectal cancer, liver cancer and esophageal cancer. Cancer has become the leading cause of human death in China.
With regard to treatment of cancers, scientists have carried out a lot of research work. New anticancer drugs are discovered continuously. Currently, there are more than 20 kinds of cancers, cure rates of which are above 30%. The research of drug action mechanism at sub-cellular and molecular level largely expands the research in the application of anti-cancer drugs. The rapid development of cell kinetics, pharmacokinetics and immunological research makes the drug screening, dose titeration, and determination of route of administration become more and more mature. Now treatments of malignant tumors have achieved very good therapeutic effects by means of combination therapy, high-dose intermittent therapy, adjuvant chemotherapy, and therapy in combination with traditional Chinese medicine. Nowadays the means for treatment of cancer are mainly surgery therapy, radiation therapy, chemotherapy, traditional Chinese medicine therapy and immunotherapy etc. The choice of anti-cancer drugs, toxicity and drug resistance etc. affect the efficacy since the anti-cancer drugs can kill not only tumor cells, but also cells of normal tissues, especially quickly proliferative hematopoietic cell in bone marrow and stomach intestinal cells. This limits doses of anti-cancer drugs, and reduces immune function in patients. Worse still, it can result in the failure of the treatment as a result of the unbearable gastrointestinal reactions which force the patients to discontinue the treatment. Anti-cancer drugs can kill cancer cells, but also have cytotoxicity. So, it is always the goal of scientists to find a drug which can treat cancer and have no or little harm to human. Recently, researches on the relationship between TRPC6 protein as one member of subfamily of transient receptor potential channels and the change of intracellular calcium concentration, development of tumor, changes of tumor cell cycle has made new progress. TRPC6 is expected to become a new target for cancer therapy.
Transient receptor potential channel (TRPC) is a non-selective cation channel protein family commonly found in the cell membrane, and plays an important role in mediating sensory conduction, cell signal transduction and regulation of development etc. Currently, it is one of hotspots in research field of ion channels. TRP channel proteins are a large family, and are widely expressed in a variety of organisms, tissues and cells. As far as mammalian TRP channels are concerned, this family includes seven interrelated sub-families: TRPC, TRPV, TRPM, TRPN, TRPA, TRPP and TRPML, each of which in turn comprises a number of family members. The previous research on TRP ion channels was restricted to the nervous system. Recent studies have shown that TRP channels play an important role not only in cellular signal transduction, mediating nociception etc. in the body, but also in tumor occurrence and development. The family has a stabilizing and regulating effect on cells, its increased expression promotes growth of malignant tumors.
TRPCs, namely the traditional TRP channel, are the first TRP channel proteins which are isolated and researched. TRPC has 7 subtypes, namely TRPC (1˜7), wherein TRPC3 and TRPC6 are very similar in structure and function, and the identity of amino acids is as high as 70%-80%. Besides, their pharmacological properties and signal regulating functions are also similar. They are more representative in TRPC subfamily, and are two subtypes concerned in the current international research. And TRPC6 is considered as the most selective channel protein. Human TRPC6 locates on chromosome 11q212q22, has a total of 132,287 bases (gene pool: NC000011), and contains 13 exons. The mRNA as transcription product of TRPC6 contains 4,564 bases, wherein the 1-427 positions are 5′ untranslated region, the 428-3,223 positions are coding region, the 3,224-4,564 positions are 3′ untranslated region (gene pool: NM004621). TRPC6 can be specifically activated by phospholipase C (PLC), subsequently lead the ligand to bind to the membrane receptor by G-protein coupled receptor (GPCR) mediated signal transduction pathway, then activate phospholipase C to generate 1,4,5-inositol triphosphate, which binds to a receptor to promote the Ca2+ release from the endoplasmic reticulum. TRPC6 is a non-selective cation channel through which calcium ions can pass, and is expressed in many tissues. It can directly be activated by the second messenger diacylglycerol enzymes, subsequently intracellular calcium flux is changed by phosphorylation regulation of particular tyrosine/serine. The increase of intracellular free Ca2+ activates some protein phosphatases, resulting in the phosphorylation of substrate proteins. The external signals are enlarged by cascade amplification, then enter the nucleus and affect the DNA replication, leading to malignant transformation of cells as well as proliferation and differentiation of tumor cells. Intracellular Ca2+ directly involves in the regulation of growth, invasion, metastasis, and differentiation of tumors. Therefore, TRPC6 inhibitors are expected to become new drugs to treat cancer. However, there are few reports about TRPC6 inhibitors.
In recent years, scientists have conducted a series of studies on the relationship between TRPC6 and human tumors. The results demonstrate that TRPC6 is closely associated with the higher incidence of gastric cancer, liver cancer, esophageal cancer and so on. David G. W. reported the TRPC3 and TRPC6 inhibitors in 2013. The compounds synthesized by them have IC50 values that can reach nanomolar order for hTRPC3 and hTRPC6. However, as for animal experiments, the series of drugs were found to have low oral bioavailability and unduly high in vivo clearance rate. Even after a series of structural modification, the people still cannot find a balance point to make both the activity and oral bioavailability arrive at a good level.
Through a large number of screening, we found that compound 1 has excellent TRPC6 inhibitory effect and is a potential antitumor drug.